Oxygen chemisorption and initial oxidation of Cr(110)

The initial stages of the interaction of oxygen with a Cr(110) surface have been investigated at 300 K by LEED, AES, electron energy loss spectroscopy (ELS), secondary electron emission spectroscopy (SES) and work-function change measurement (Δφ). In the exposure region up to 2 L, the clean-surface ELS peaks due to interband transition weakened and then disappeared, while the ～5.8 and 10 eV loss peaks attributed to the O 2p → Cr 3d transitions appeared, accompanied with a work-function increase (Δφ = +0.19 eV at2L). In the region 2–6 L the work function decreased to below the original clean-surface value (Δφ_min = ?0.24 eV at6L), and five additional ELS peaks were observed at ～2, 4, 11, 20 and 32 eV: the 2 and 4 eV peaks are ascribed to the ligand-field d → d transitions of a Cr³⁺ ion, the 11 eV peak to the O 2p → Cr 4s transition, the 20 eV peak to the Cr 3d → 4p transition of a Cr³⁺ ion and the 32 eV peak probably to the Cr 3d → 4f transition. A new SES peak at 6.1 eV, being attributed to the final state for t he 11 eV ELS peak, was observed at above 3 L and identified as due to the unfilled Cr 4s state caused by charge transfer from Cr to oxygen sites in this region. In the region 6–15 L the work function increased again (Δφ_max = +0.32 eV at15 L), the 33 and 46 eV Auger peaks due to respectively the M_2,3(Cr)L_2,3(O)L_2,3(O) cross transition and the M_2,3VV transition of the oxide appeared and the 26 eV ELS peak due to the O 2s → Cr 4s transition was also observed. Above 10 L, the ELS spectra were found to be practically the same as that of Cr₂O₃. Finally, above 15 L, the work function decreased slowly (Δφ = +0.13 eV at40L). From these results, the oxygen interaction with a Cr(110) surface can be classified into four different stages: (1) dissociative chemisorption stage up to 2 L, (2) incorporation of O adatoms into the Cr selvedge between 2–6 L, (3) rapid oxidation between 6–15 L leading to the formation of thin Cr₂O₃ film, and (4) slow thickening of Cr₂O₃ above 15 L. The change in the Cr 3p excitation spectrum during oxidation was also investigated. The oxide growth can be interpreted on the basis of a modified coupled current approach of low-temperature oxidation of metals.     

ELS study on initial oxidation of Ni(100) surfaces

Electron energy loss spectra of clean and oxygen-covered Ni(100) surfaces were observed with concomitant measurements of LEED, work function change, and Auger peak height ratio O(KL_{2, 3}L_{2, 3})/Ni(L_{2, 3}VV). The observed electronic transitions are interpreted on the basis of primary election energy dependence, and of comparison with the loss spectrum for a UHV-cleaved NiO(100) surface and optical data of Ni. The observed loss peaks at 9.1, 14, and 19 eV in the clean surface spectrum are ascribed to the bulk plasmon of the 4s electrons, the surface plasmon, and the bulk plasmon of the coupled 3d + 4s electrons, respectively, and the weak but sharp peak at 33 eV is tentatively attributed to the localized many-body effect in the final state. Three oxygen-derived peaks at 6.0, 8.0, and 10.3 eV in the low oxygen exposure region (?4 L) are ascribed to the O 2p(e) → Ni 3d, O 2p(a₁) → Ni 3d, and O 2p → Ni 4s transitions, respectively. In the high oxygen exposure region (?50 L), the spectra become quite similar to that of the UHV-cleaved NiO(100) surface. The oxidation process consistent with LEED, Auger peak height ratio and work function change measurements is discussed.     

Initial oxidation stages on FeCr(100) and FeCr(110) surfaces

Simultaneous LEED and AES are used to follow early stages of oxidation of monocrystalline FeCr(100) and (110) between 700 and 900 K in the oxygen pressure range 10^?9–10^?6 Torr. A chromium-rich oxide region at the alloy/oxide interface is observed, which exhibits different surface structures on oxidized FeCr(100) and FeCr(110). The chromium concentration in this initially formed oxide film is found to be enhanced by low oxygen pressures or high temperatures. During further oxidation different behaviours are observed on FeCr(100) and FeCr(110), which are explained by assuming different ion permeabilities through the initial chromium rich oxide regions on the two surface planes. On FeCr(110) surfaces oxidation is initiated on chromium enriched (100) facets at 800 K or below. At 900 K a film consisting of rhombohedral Cr₂O₃ or (Fe, Cr)₂O₃ is epitaxially growing with its (001) plane parallel to the alloy (110) face. On FeCr(100) surfaces the chromium rich oxide region next to the substrate is of fcc type. As soon as the diffusion of iron from the alloy to the gas/oxide interface is observable, a spinel type oxide is formed and connected with the location of iron in tetrahedral lattice sites. Closer to the fcc lattice the spinel oxide consists of FeCr₂O₄ or a solid solution of FeCr₂O₄ and Fe₃O₄ whereas next to the gas phase the oxide is pure Fe₃O₄.     

Angle-resolved AES study on initial oxidation of Ni (100) surface

The Ni-M_2,3VV Auger electron angular distributions have been measured from oxygen-adsorbed Ni (100) surfaces and from cleaved NiO (100) clean surfaces. Significant variations in the angular profiles have been observedin the second reaction stage of initial oxidation process. Present results support the island growth model of NiO layers. It is also shown that the new information on the ratio of domain area of NiO to that of c (2 × 2)-O on Ni (100) surface can be obtained by angle-resolved AES method.     

A LEED-AES study of the oxidation of cr(110) and Cr(100)

The initial stages of the oxidation of (110) and (100) chromium surfaces have been studied using low energy electron diffraction and Auger electron spectroscopy. The low energy Auger electron peaks were tentatively explained in terms of different chemical states. Thus, the clean chromium surface, the surface covered by chemisorbed oxygen and the chromium oxide surface could be associated with the occurrence of different peaks. The intermediate oxygen chemisorption structures observed at oxidation, have been characterized with respect to symmetry and accurate unit cell dimensions. Lattice parameters were found to range from those of the substrate chromium metal to those of chromium sesquioxide. On the (110) face, the lattice parameter change was observed to be largest in the [11̄0] direction. The observations are in fair agreement with current concepts of misfitting crystalline surface layers.     

All-optical determination of initial oxidation of Si(100) and its kinetics

By comparison of measured and ab initio calculated surface optical spectra we demonstrate that two main oxidation processes initially occur after dissociation of oxygen molecules, forming in both cases Si–O–Si entities: (i) breaking of Si dimers by incorporation of oxygen atoms; (ii) incorporation into the silicon backbonds. The kinetics up to half-monolayer coverage is determined, and explained in terms of Langmuir-like adsorption mechanisms with different probabilities.     

LEED and ELS study of the initial oxidation of Pd(100)

Oxygen-Pd(100) interactions were studied by means of LEED, AES and ELS to characterize the initial oxidation and elucidate the relationship between surface-oxide reconstructions and bulk oxidation. Five oxygen-related LEED patterns were identified and classified into three types. First, the chemisorption structures included the usual p(2 × 2) which was replaced by a c(2 × 2) upon increased exposure to oxygen at room temperature. Second, surface-oxide reconstructions included a p(5 × 5) which formed above 200°C and which was replaced by a (√5 × √5) R27° above 300°. These two structures are attributed to PdO(110) and PdO(001) coincidence lattices, respectively. Above approximately 500°C the oxygen was removed due to thermal desorption and possibly absorption, but a multilayer oxide, distinct from PdO, could form upon oxygen exposure above 800°C. The ELS study indicated that this last species had significantly different electronic properties than that of the other structures. It is quite plausibly a Si-stabilized oxide as has been suggested recently by Niehus and Comsa for the Pt-oxygen system.     

The initial oxidation of Cu(100) single crystal surfaces: an electron spectroscopic investigation

The total energy distribution of electrons emitted from clean Cu(100) and oxygen covered surfaces is analysed. A primary electron energy of 400 eV enabled the investigation of characteristic losses (ELS), Cu MVV Auger transitions and true secondary electrons in a single spectroscopic run. Oxygen exposure up to 10⁸ L at elevated temperature (~400 K) results in a Cu density of states (DOS) strongly affected by O(2p) electrons. The Auger lines of Cu, atomic-like for clean surfaces, reveal DOS effects after some 10⁷ L oxygen exposure: all MVV transitions shift down by ~2 eV in spite of a fixed M₂₃ level; the M₂₃VV Auger line splitting is vanishing due to a broadened valence band maximum allowing the deexcitation of the final two-hole state of intraatomic transitions. Heating the oxygen covered crystal to 820 K is accompanied by the removal of much surface oxygen and an electronic state resembling an earlier oxidation state without DOS effects in the Cu Auger spectrum.     

Kinetic hindrance during the initial oxidation of Pd(100) at ambient pressures

The oxidation of the Pd(100) surface at oxygen pressures in the 10(-6) to 10(3) mbar range and temperatures up to 1000 K has been studied in situ by surface x-ray diffraction (SXRD). The results provide direct structural information on the phases present in the surface region and on the kinetics of the oxide formation. Depending on the (T,p) environmental conditions, we observe either a thin (sqrt[5]xsqrt[5])R27 degrees surface oxide or the growth of a rough, poorly ordered bulk oxide film of PdO predominantly with (001) orientation. By either comparison to the surface phase diagram from first-principles atomistic thermodynamics or by explicit time-resolved measurements we identify a strong kinetic hindrance to the bulk oxide formation even at temperatures as high as 675 K.     

The initial oxidation of molybdenum: III. Oxide nucleation rates on (100) and (110) molybdenum surfaces

The induction periods for the nucleation of epitaxial oxide on (100) and (110) surfaces of molybdenum have been determined. The role of impurities has been investigated by AES. It is found that only gross amounts of impurity are likely to be significant. Under isobaric conditions the activation energy associated with nucleation is 0.11 ± 0.2 MJ mole^?1 on both faces between 700 and 1050 K. At 850 K the induction rate is proportional to the pressure. Under isoposal conditions on the (110) surface an activation energy of 0.22 ± 0.03 MJ mole^?1 is found.     

Vibrational study of the initial stages of the oxidation of Si(111) and Si(100) surfaces

Using high-resolution electron energy loss spectroscopy (EELS) the vibrations of Si(111) and Si(100) surfaces in the early stages of oxidation have been investigated. Three different stages of oxidation, the last being the formation of a thin layer of vitreous SiO₂ are identified when the surfaces are held at a temperature of 700K during the exposure with molecular oxygen. We show that also the first two stages involve atomic oxygen in bridging positions between silicon atoms. Small exposures at low temperatures (100 K) produce vibrational features of a different, possibly molecular, species. For higher exposures at the same temperature the spectrum again develops the characteristics of atomic oxygen and the molecular species eventually disappears. Exposure at room temperature leads to a mixture of atomic and molecular oxygen for smaller exposures and to purely atomic oxygen for exposures greater than 10² L. At room temperature even exposures as high as 10¹¹ L do not produce the spectrum of vitreous SiO₂. The same is found for the natural, room temperature grown, oxide layer on silicon wafers which we have studied by introducing the sample into the spectrometer through an air-lock. Annealing of the wafer to 700 K produced the characteristic spectrum of vitreous SiO₂. The results are discussed in comparison with previous work.     

Cesium enhanced oxidation of Ni(100)

It has been found by LEED and work function measurements that the presence of a fractional monolayer of Cs increased the oxidation rate of a Ni(100) surface by about thirteenfold. The NiO formed in two crystallographic orientations depending on the initial Cs coverage. Work function measurements suggest that in the enhanced oxidation the Cs atoms remained on top of the oxide.     

Oxygen adsorption and oxide formation on Cr(100) and Cr(110) surfaces

The adsorption of oxygen on (100) and (110) clean chromium surfaces was studied by electron diffraction (LEED and RHEED) and scanning microscopy. The beginning of the oxidation process was investigated using LEED technique. The successive patterns, observed with increasing oxidizing atmosphere, allow us to propose an oxidation mechanism for these chromium surfaces. For both orientations, rhombohedral chromium oxide grows on the surface, as shown by RHEED diagrams. The behaviour of the two planes is markedly different: the oxide film is polycrystalline on the (100) surface, while the pseudo six-fold symmetry of the (110) plane permits a parallel oriented growth.     

Electron and positron work functions of Cr(100)

We report measurements of the electron and positron work functions of submonolayer contaminated single crystal surfaces of Cr(100) in ultra high vacuum. The positron work function ø₊ is obtained by measuring the spectrum of slow positrons reemitted by the Cr(100) surface when it is bombarded with keV energy positrons. The electron work function ø_- is measured relative to Al(100) by comparing the target biases at which the slowest emitted positrons are recollected by the target. We obtain ø₊ = ?1.76(10) eV and ø_- = 4.46(6) eV for our Cr(100) surface using the value ø_- = 4.41(3) eV for Al(100) reported by Grepstad, Gartland and Slagsvold. The ø₊ value is in agreement with the ?2.2 eV calculated by Nieminen and Hodges. The positronium work function for Cr implied by these results is ?4.10(10) eV; the positronium negative ion (Ps^-) work function for this surface is calculated to be + 0.37(7) eV. A search for Ps^- showed that at a 90% confidence level less than one in 10³ thermalized positrons reaching the Cr surface are emitted as Ps^-. The slow positron emission spectrum was observed not to change over the 70–300 K range in contrast to recent theoretical predictions.     

Search for ferromagnetism in ultrathin epitaxial films: Cr/Au(100), Cr/Cu(100), and Fe/Cu(100)

The surface magneto-optic Kerr effect (SMOKE) technique was used to search for ferromagnetism in monolayer-range films of Cr and Fe grown on Au(100) and Cu(100). The growth modes were characterized using low energy electron diffraction (LEED) and Auger electron spectroscopy. The fcc structure of Cr could not be stabilized on Cu(100). Ferromagnetism was not observed for the Cr/Au(100) films at temperature above 100 K. Ferromagnetism also was not observed for fcc Fe/Cu(100) grown at room temperature; but for growth at >150°C, a ferromagnetic, metastable state was observed for the top layer of the Fe film, in the absence of bulk ferromagnetism. The ferromagnetic Fe/Au(100) system was used to establish the sensitivity of the approach.     

Adsorption of triplet O(2) on Si(100): the crucial step in the initial oxidation of a silicon surface

It has long been understood that a precursor mediated chemisorption is a significant part of the dynamics for the adsorption of O(2) on Si(100), which is a much studied model system of surface reaction with considerable technological relevance. However, theoretical studies on the interaction between O(2) and Si(100) have been focused on the excited singlet state of O2 and unable to explain the observations in surface scattering experiments. We demonstrate by first principles calculations that such a focus is misplaced. In reality, triplet O(2) can also react with Si(100), after overcoming small barriers, and its reaction paths provide a full account for experiments. Our results highlight the important role played by triplet O(2) in surface oxidation.